SIP registration: definition and a practical example
Short answer: SIP registration is the process in which a phone, PBX or trunk sends a REGISTER message to the registrar to report its current address and be able to receive calls for its account.
In Portuguese: Registro SIP
What is SIP registration?
SIP registration is how a phone, PBX or SIP trunk tells the registrar where it is. The device sends a REGISTER request, defined in RFC 3261, with its account and its IP address and port. The registrar keeps that binding for a period, given in the Expires field, and forwards calls meant for the account to that address.
The usual flow has four steps: the device sends REGISTER; the registrar answers 401 Unauthorized with a challenge; the device resends the REGISTER with a response calculated from the password; the registrar confirms with 200 OK. Before the period runs out, the device registers again. If the registration expires and is not renewed, inbound calls stop arriving.
Many devices also use SIP OPTIONS (RFC 3261, section 11) as a keep-alive: a periodic query that, when answered with 200 OK, shows the other side is alive. The regular traffic also keeps the NAT mapping open on the router, which avoids problems with SIP ALG and firewalls.
Registration and OPTIONS test the signaling up to the registrar. They do not go through the carrier’s routing, the channel limit, the codecs or the audio.
In practice
What each check proves about a trunk:
| Check | Expected response | Proves that |
|---|---|---|
| REGISTER | 200 OK, with Expires of 3600 s, for example | Credentials accepted and registrar reachable |
| OPTIONS | 200 OK | The other side responds to signaling |
| Real call | 200 OK and audio in both directions | Calls complete with voice |
A trunk can pass the first two rows all day and fail the third, for example with 503 Service Unavailable when the channels run out. On the PBX, the trunk keeps showing as registered throughout the failure.
How Monitor uses this
A trunk can be registered and still not complete calls, so the test is a real call. When 2N5’s own calling account cannot register, the test is marked “monitoring blind” and is never blamed on the customer’s line. See SIP trunk monitoring.
Frequently asked questions
Does a registered trunk mean it is working?
Not necessarily. Registration shows that the PBX can talk to the carrier's registrar and that the credentials were accepted. Calls can still fail because of a channel limit, an incompatible codec, wrong routing at the carrier or audio blocked by NAT. Only a real call, with a final SIP response and audio in both directions, shows that the trunk works.
What is SIP OPTIONS for?
It is a request defined in RFC 3261, section 11, for asking another device which capabilities it supports. In practice, PBXs and SBCs send OPTIONS periodically as a keep-alive: if they get 200 OK, they consider the other side up. Sending it regularly also keeps the NAT mapping open on the router. An answered OPTIONS does not prove that calls complete.